Yup, you should get a JUMVEA that states the mileage in KM when it was put up for auction and your first MOT should have the mileage in KM on it (mine does). When SVA get thier hands on it they will stick a black stocker over the K on the speedo and convert it from KM to Miles. Your mileage can't be clocked back so that it shows the mileage in miles because this would be illegal, so the first 23,000 will be KM and every mile after that will be miles.
So 14,291 miles for your 23,000 km add to that 17,000 miles and the car has done just over 31,000 miles.
Took me a while to get my head round it at first as well